Polymer sustained alignment (PSA) liquid crystal display elements have a structure in which a polymer structure is formed in a cell in order to control the pretilt angle of liquid crystal molecules, and are expected to become next-generation liquid crystal display elements because of their high-speed responsiveness and high contrast.
Such a PSA liquid crystal display element is produced by injecting a polymerizable composition containing a liquid crystal compound and a polymerizable compound between substrates, and polymerizing the polymerizable compound while aligning liquid crystal molecules by applying a voltage, thus fixing the alignment of the liquid crystal molecules. It is known that the causes of image sticking, which is display damage of this PSA liquid crystal display element, include impurities and a change in the alignment of liquid crystal molecules (i.e., change in the pretilt angle).
Image sticking due to impurities is caused by a polymerizable compound that remains because of incomplete polymerization and a polymerization initiator added for the purpose of accelerating the progress of the polymerization. Therefore, it is necessary to minimize the amount of polymerizable compound remaining after the polymerization and to suppress the amount of polymerization initiator added. For example, when a large amount of polymerization initiator is added in order to completely carry out the polymerization, a voltage-holding ratio of the resulting display element is decreased by the residual polymerization initiator, resulting in an adverse effect on the display quality. On the other hand, when the amount of polymerization initiator used in the polymerization is reduced in order to suppress the decrease in the voltage-holding ratio, the polymerizable compound remains because the polymerization does not completely proceed, and thus image sticking due to the residual polymerizable compound inevitably occurs. In order to suppress the amount of residual polymerizable compound by completely curing (i.e., hardening) a polymerizable compound with a small amount of polymerization initiator added, a method of applying a large amount of energy may be employed, specifically, for example, strong ultraviolet light may be applied for a long time during the polymerization. In this case, however, in addition to an increase in the size of a production apparatus and a decrease in the production efficiency, for example, degradation of a liquid crystal material due to ultraviolet light may be caused. Thus, in existing liquid crystal compositions containing a polymerizable compound, it is difficult to reduce both the amount of residual uncured polymerizable compound and the amount of residual polymerization initiator at the same time. Accordingly, it has been desired to develop a liquid crystal composition containing a polymerizable compound that is completely polymerized without using a polymerization initiator.
Meanwhile, it is also known that image sticking may be caused by a change in the pretilt angle of liquid crystal molecules in a liquid crystal composition containing a polymerizable compound. Specifically, in the case where a polymer obtained by curing a polymerizable compound is flexible, when a display element is fabricated and a certain pattern is continued to be displayed for a long time, the structure of the polymer changes, resulting in the change in the pretilt angle. Since such a change in the pretilt angle may become a cause of image sticking, it is necessary to use a polymerizable compound capable of forming a polymer which has a rigid structure and whose structure does not change.
Hitherto, in order to prevent image sticking by improving the rigidity of a polymer, a display element has been constituted by using a polymerizable compound having a structure such as a 1,4-phenylene group having only a ring structure and polymerizable functional groups (refer to Japanese Unexamined Patent Application Publication No. 2003-307720) and a display element has been constituted by using a polymerizable compound having a biaryl structure (refer to Japanese Unexamined Patent Application Publication No. 2008-116931). However, these polymerizable compounds have low compatibility with liquid crystal compounds, and thus deposition of the polymerizable compounds occurs when a liquid crystal composition is prepared. Thus, it is difficult to apply these polymerizable compounds to a liquid crystal composition suitable for practical use.
In addition, in order to prevent image sticking by improving the rigidity of a polymer, it has been proposed that a display element is constituted by using a mixed liquid crystal composition containing a bifunctional polymerizable compound and a trifunctional or higher functional polymerizable compound such as dipentaerythritol pentaacrylate or dipentaerythritol hexaacrylate (refer to Japanese Unexamined Patent Application Publication No. 2004-302096). However, since dipentaerythritol pentaacrylate and dipentaerythritol hexaacrylate have no ring structure in their molecules, the affinity of these polymerizable compounds with liquid crystal compounds is weak and a force that controls the alignment is also weak. Accordingly, sufficient alignment stability is not achieved. Furthermore, it is essential to add a polymerization initiator in polymerization of these polymerizable compounds. Unless a polymerization initiator is added, these polymerizable compounds remain after the polymerization.
As described above, it is difficult to satisfy characteristics desired in polymerizable-compound-containing liquid crystal compositions, such as image sticking characteristics of a display element, alignment stability, stability of the composition having such a property that no deposition is produced, and the production efficiency in the fabrication of a PSA liquid crystal display element, and this difficulty inhibits practical application of this display element.